Voltage regulators often use a shunting element to maintain a stable voltage from an incoming power source providing a higher voltage than that required by an electrical load. Such voltage regulators use a shunting element such as a transistor to divert extra current from the incoming power source when voltage exceeds the desired regulated level. Shunting regulators provide simple, effective low cost voltage regulation that is adequate for many electronic devices, and are therefore ubiquitous in low power electronics designs. For example, shunting voltage regulators are used with electronic protective devices such as circuit breakers and motor overload relays.
Shunting voltage regulation may include linear or switching type shunts. Linear shunts operate by dissipating a variable amount of power to maintain a stable regulated voltage. The power dissipated by the shunting element is therefore the product of the shunted current and the regulated voltage. The shunted current is set to the difference between the incoming supply current and the current consumed by the load on the regulated voltage. The shunt current is linearly related to the load current to linearly regulate the output voltage.
In contrast, switching shunt regulators operate by switching the shunting element between on and off states. With the shunting element off, the shunt current is zero. When the shunting element is on, the shunt current is the total supply current provided by the incoming power source. The instantaneous power dissipation is decreased because the voltage across the shunting element is reduced from the load voltage to a minimum based on the conduction resistance of the shunting element. This reduces the power dissipated in the shunting element overall, allowing a switching shunt regulator to achieve a lower power dissipation and higher efficiency than a linear shunt regulator.
FIG. 1 shows a typical voltage regulation system 10 having a switching shunt circuit. The voltage regulation system 10 includes an incoming direct current power source 12 which is coupled to a switching shunt circuit 14. The shunt circuit 14 regulates the power output from the direct current power source 12 to a voltage regulator circuit 16 which is coupled to a voltage load 18. The switching shunt supply regulation circuit 10 has a shunt transistor 20 that is coupled to a charging capacitor 22 in the voltage regulation circuit 16. The incoming power from the direct current power source 12 is accumulated in the charging capacitor 22 which creates a low pass filter for the supply voltage 12. When the shunt transistor 20 is on, the power delivered to the voltage load 18 is provided by discharging the capacitor 22, which is charged when the shunt transistor 20 is off.
Simple control of the shunt transistor 20 may be achieved with an analog control circuit that is coupled to a gate 24 of the shunt transistor 20 to switch the shunt transistor 20 on or off. Comparators, operational amplifiers or similar analog elements may establish a nominal supply voltage. With a switching shunt regulator, a hysteresis band around the nominal supply voltage determines transition points for turning the shunt transistor 20 on and off. When the supply voltage from the regulator circuit 16 reaches or falls below the hysteresis minimum, the control circuit turns the shunt transistor 20 off allowing the direct current power source 12 to be connected to the charging capacitor 22 thus accumulating energy in and increasing the charge of the charging capacitor 22. When the supply voltage from the regulator circuit 16 reaches or exceeds the hysteresis maximum the shunt transistor 20 is turned on, the direct current power source 12 is disconnected from the charging capacitor 22, and the load 18 is powered by the discharging of charging capacitor 22.
With the continued trend toward integration of digital controls into electronic devices, digital logic has replaced analog control components. Digital electronics devices such as a programmable digital controller may achieve the functions of the operational amplifiers and comparators used to control the shunt operation in an analog design. For example, a microcontroller can monitor the regulated supply voltage via digital sampling and make regulator shunt control decisions based on the sampling. A simple state machine routine executed by a digital controller may be used to control the shunt transistor 20. As is known, a state machine is a low processing programmable routine that makes decisions between states. In this case, the state machine has a first state where the shunt transistor 20 is on and a second state where the shunt transistor 20 is off. The decision to transition between states is made depending on the sampled regulated supply voltage.
Self-powered devices such as circuit breakers and overload relays using a digital controller have some intrinsic design constraints. Digital controllers require power to sample the supply voltage and execute routines such as the state machine to control the shunting element. Further, power is required to maintain the shunt transistor 20 in either an on or off state. However, reducing the power consumed by the controller can help increase the operational range of the controller or help improve measurement accuracy by reducing the power burden on the regulated voltage circuit that provides operational power to the controller. Maximizing the operation of the digital controller in low power consumption modes helps to reduce power consumption and achieve the benefits associated with reduced power consumption.
For digital control of the switching shunt circuit, low power operation may be achieved simply by reducing the frequency of updating the input voltage measurements to the state machine. However, the tradeoff associated with reducing the update frequency is that with less frequent sampling of the supply voltage, the response time of controlling the shunting element to prevent a voltage approaching or crossing beyond the hysteresis limits is accordingly increased.
The self-powered protective device may also be required to withstand or operate during conditions beyond the normal operational range of the electrical load. For example, during a short circuit event, the current measured by the protective device may exceed the normal current of the protected load by several orders of magnitude. This can correspond to a charging slew rate of the power supply voltage also orders of magnitude faster than during normal operation of the device. This introduces an additional requirement of the regulated power supply to respond to a significantly faster rise in the power supply voltage to protect the device electronics from an over voltage condition. Overvoltage protection in the form of a zener diode may be added to the typical shunting power supply circuit but this does not address the issue of a rapid reaction to an overcurrent condition. When the current measured by the protective device may exceed the normal current by several orders of magnitude the zener diode is required to dissipate power several orders of magnitude above normal operation. This creates a constraint to withstand this power dissipation and to use zener diode components with the necessary power dissipation rating which may not exist. Current digital controllers must therefore sample the regulated voltage frequently in order to protect against massive overcurrents. Such capabilities require greater power consumption from the controller.
Thus, a need exists for a digitally controlled power regulation circuit that minimizes energy consumed by the controller while allowing for rapid switching of the shunt element. There is a further need for a power regulation circuit that allows for rapid transitions between the on and off state of the shunting element. There is also a need for a power regulation circuit that detects overcurrents and turns on the shunt element to protect loads independent of the sampling speed of the controller thereby minimizing power consumption.